JP5630322B2 - High-tensile steel plate with excellent toughness and manufacturing method thereof - Google Patents
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- 229910000831 Steel Inorganic materials 0.000 title claims description 100
- 239000010959 steel Substances 0.000 title claims description 100
- 238000004519 manufacturing process Methods 0.000 title claims description 9
- 239000000463 material Substances 0.000 claims description 46
- 229910000734 martensite Inorganic materials 0.000 claims description 35
- 230000009466 transformation Effects 0.000 claims description 32
- 238000001816 cooling Methods 0.000 claims description 22
- 238000003466 welding Methods 0.000 claims description 18
- 239000000203 mixture Substances 0.000 claims description 13
- 229910052748 manganese Inorganic materials 0.000 claims description 8
- 238000003303 reheating Methods 0.000 claims description 8
- 229910052759 nickel Inorganic materials 0.000 claims description 6
- 229910052804 chromium Inorganic materials 0.000 claims description 5
- 230000001186 cumulative effect Effects 0.000 claims description 5
- 229910052757 nitrogen Inorganic materials 0.000 claims description 4
- 229910052799 carbon Inorganic materials 0.000 claims description 3
- 229910052750 molybdenum Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 239000012535 impurity Substances 0.000 claims description 2
- 229910052698 phosphorus Inorganic materials 0.000 claims description 2
- 229910052717 sulfur Inorganic materials 0.000 claims description 2
- 229910052738 indium Inorganic materials 0.000 claims 1
- 238000000034 method Methods 0.000 description 26
- 229910001566 austenite Inorganic materials 0.000 description 16
- 238000010791 quenching Methods 0.000 description 14
- 230000000171 quenching effect Effects 0.000 description 12
- 229910000859 α-Fe Inorganic materials 0.000 description 10
- 230000000694 effects Effects 0.000 description 9
- 238000010438 heat treatment Methods 0.000 description 9
- 229910001563 bainite Inorganic materials 0.000 description 8
- 229910052761 rare earth metal Inorganic materials 0.000 description 7
- 238000005496 tempering Methods 0.000 description 7
- 230000007423 decrease Effects 0.000 description 6
- 238000009863 impact test Methods 0.000 description 6
- 150000002910 rare earth metals Chemical class 0.000 description 6
- 238000005096 rolling process Methods 0.000 description 6
- 230000015572 biosynthetic process Effects 0.000 description 5
- 238000001556 precipitation Methods 0.000 description 5
- 238000011282 treatment Methods 0.000 description 5
- 238000005098 hot rolling Methods 0.000 description 4
- 239000011159 matrix material Substances 0.000 description 4
- 150000004767 nitrides Chemical class 0.000 description 4
- 238000012360 testing method Methods 0.000 description 4
- 238000005516 engineering process Methods 0.000 description 3
- 238000005242 forging Methods 0.000 description 3
- 238000002844 melting Methods 0.000 description 3
- 230000008018 melting Effects 0.000 description 3
- 238000009864 tensile test Methods 0.000 description 3
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- 229910000532 Deoxidized steel Inorganic materials 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 238000004364 calculation method Methods 0.000 description 2
- 238000004090 dissolution Methods 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 238000005259 measurement Methods 0.000 description 2
- 229910052751 metal Inorganic materials 0.000 description 2
- 239000002184 metal Substances 0.000 description 2
- 239000002244 precipitate Substances 0.000 description 2
- 239000002994 raw material Substances 0.000 description 2
- 238000004088 simulation Methods 0.000 description 2
- 230000000087 stabilizing effect Effects 0.000 description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 2
- 238000012935 Averaging Methods 0.000 description 1
- 229910052684 Cerium Inorganic materials 0.000 description 1
- 238000003723 Smelting Methods 0.000 description 1
- 229910000746 Structural steel Inorganic materials 0.000 description 1
- ATJFFYVFTNAWJD-UHFFFAOYSA-N Tin Chemical compound [Sn] ATJFFYVFTNAWJD-UHFFFAOYSA-N 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 238000005275 alloying Methods 0.000 description 1
- 239000010953 base metal Substances 0.000 description 1
- 230000000052 comparative effect Effects 0.000 description 1
- 238000009749 continuous casting Methods 0.000 description 1
- 229910052802 copper Inorganic materials 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000004927 fusion Effects 0.000 description 1
- 239000007789 gas Substances 0.000 description 1
- 238000010191 image analysis Methods 0.000 description 1
- 230000001771 impaired effect Effects 0.000 description 1
- 238000005304 joining Methods 0.000 description 1
- 229910052746 lanthanum Inorganic materials 0.000 description 1
- 150000001247 metal acetylides Chemical class 0.000 description 1
- 229910052758 niobium Inorganic materials 0.000 description 1
- 229910052760 oxygen Inorganic materials 0.000 description 1
- 229910001562 pearlite Inorganic materials 0.000 description 1
- 238000012545 processing Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000001737 promoting effect Effects 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 229920006395 saturated elastomer Polymers 0.000 description 1
- 239000006104 solid solution Substances 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- 230000035882 stress Effects 0.000 description 1
- 150000003568 thioethers Chemical class 0.000 description 1
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Description
本発明は、船舶や海洋構造物、圧力容器、ペンストックなどの鋼構造物に用いられる高張力鋼板とその製造方法に関し、特に板厚が30mm以上、降伏強度が630MPa以上で、母材の強度・靭性に優れるだけでなく、溶接熱影響部の靭性にも優れる高張力鋼板とその製造方法に関するものである。本発明において、降伏強度とは、降伏点(YP)もしくは0.2%耐力(YS)を指す。 The present invention relates to a high-strength steel plate used for steel structures such as ships, offshore structures, pressure vessels, and penstocks, and a method for producing the same. Particularly, the thickness is 30 mm or more, the yield strength is 630 MPa or more, and the strength of the base material. -It relates to a high-tensile steel sheet not only excellent in toughness but also excellent in the toughness of the weld heat-affected zone and its manufacturing method. In the present invention, the yield strength refers to the yield point (YP) or 0.2% yield strength (YS).
船舶や海洋構造物、圧力容器などの各種鋼構造物は、板厚が厚い鋼板等の鋼材を溶接して接合し、所望の形状や構造に仕上げるのが普通である。そのため、これらに用いられる鋼材には、安全性を確保する観点から、母材の靭性に優れるだけでなく、溶接熱影響部の靭性にも優れていることが必要とされる。 Various steel structures such as ships, offshore structures and pressure vessels are usually finished by welding steel materials such as thick steel plates and joining them to a desired shape and structure. For this reason, the steel materials used for these are required not only to be excellent in toughness of the base material but also to be excellent in toughness of the weld heat affected zone from the viewpoint of ensuring safety.
しかし、例えば板厚が30mm以上の厚鋼板は、一般に入熱量が80kJ/cm以下の多層溶接で施工されるが、この溶接熱影響部は、加熱と冷却を繰り返す複雑な熱履歴を受けるため、局所的に脆化域が発生しやすく、特に、熱影響部の中でもフェライトとオーステナイトの2相域に加熱されるボンド部(フュージョンライン(FL)ともいう。)近傍では、著しい脆化が起こることが知られている。その原因は、上記のような領域では、複数回の加熱・冷却を受ける際に何度も変態が起こり、組織が細粒化するため焼入性が低下し、靭性を低下させる島状マルテンサイトが形成される上部ベイナイト組織になるためであるといわれている。 However, for example, a thick steel plate having a thickness of 30 mm or more is generally constructed by multi-layer welding with an amount of heat input of 80 kJ / cm or less, but this welding heat affected zone receives a complicated heat history that repeats heating and cooling. The embrittlement zone is likely to occur locally, and in particular, in the heat affected zone, remarkable embrittlement occurs in the vicinity of the bond zone (also called fusion line (FL)) heated to the two-phase zone of ferrite and austenite. It has been known. The cause of this is the island martensite where transformation occurs many times when subjected to multiple heating / cooling in the region as described above, the structure becomes finer, hardenability is reduced, and toughness is reduced. It is said that this is because the upper bainite structure is formed.
上記問題に対する対策としては、鋼中にTiNを均一に微細分散させることによって、オーステナイトの粗大化を抑制すると共に、フェライトの変態核としても利用する技術が実用化されている。また、特許文献1には、Ti酸化物等を鋼中に微細分散させて、溶接熱影響部におけるオーステナイト粒の粗大化を防止し、フェライト粒を微細化することで、溶接熱影響部の靭性を向上させる技術が開示されている。しかし、これらの技術は、フェライトを母相とする比較的低強度の鋼材を対象としているため、溶接熱影響部がフェライトを含まない組織となる高強度の鋼板には、このフェライト粒微細化による溶接熱影響部の靭性向上効果を望むことはできない。 As a countermeasure against the above-mentioned problem, a technique has been put into practical use in which TiN is uniformly finely dispersed in steel to suppress coarsening of austenite and also used as a ferrite transformation nucleus. Patent Document 1 discloses that the toughness of the weld heat affected zone is obtained by finely dispersing Ti oxide or the like in the steel to prevent coarsening of the austenite grains in the weld heat affected zone and miniaturizing the ferrite grains. A technique for improving the above is disclosed. However, since these technologies are intended for relatively low-strength steel materials with ferrite as the matrix, high-strength steel sheets whose weld heat-affected zone has a structure that does not contain ferrite are produced by this refinement of ferrite grains. It cannot be expected to improve the toughness of the heat affected zone.
また、2相域加熱部、つまり最初の溶接時に融点近傍まで加熱された領域が、後続する溶接の再加熱によってフェライトとオーステナイトの2相域になる領域が最も脆化する理由は、後続の溶接による再加熱によって、オーステナイト領域にCが濃化し、この部分が冷却時に島状マルテンサイト生成を伴う上部ベイナイト組織を形成するためである。また、この領域は、粗大な島状マルテンサイトが容易に形成されるため、脆性破壊の起点となり易い。 The reason why the two-phase zone heating zone, that is, the zone heated to the vicinity of the melting point during the first welding, becomes the most brittle in the zone where the two-phase zone of ferrite and austenite becomes due to the subsequent reheating of welding. This is because C is concentrated in the austenite region due to reheating, and this portion forms an upper bainite structure accompanied by generation of island martensite during cooling. Further, in this region, since coarse island martensite is easily formed, it is likely to become a starting point of brittle fracture.
このC濃化による溶接熱影響部の靭性低下に対しては、例えば、特許文献2には、鋼を低C化および低Si化し、島状マルテンサイトの生成を抑制した上で、さらにCuを添加することで、母材強度を確保する技術が開示されている。また、特許文献3には、低C化して溶接熱影響部の靭性を向上した上で、Cu添加により強度を高める技術が開示されている。しかし、これらの技術は、時効処理によるCuの析出強化を利用して強度を高めるものであるが、多量のCu添加を必要とするため、熱間加工時に脆性を起こし易く、製造性や品質面で問題が多い。 For the toughness reduction of the weld heat affected zone due to this C enrichment, for example, in Patent Document 2, the steel is made low C and low Si, and after suppressing the formation of island martensite, Cu is further added. A technique for ensuring the strength of the base material by adding the metal is disclosed. Patent Document 3 discloses a technique for increasing strength by adding Cu after lowering C to improve the toughness of the weld heat affected zone. However, these techniques increase the strength by utilizing the precipitation strengthening of Cu by aging treatment. However, since a large amount of Cu is required, brittleness is likely to occur during hot working, and the productivity and quality are reduced. There are many problems.
また、特許文献4には、2相域加熱部の島状マルテンサイトによる靭性低下を抑制するため、母材および溶接金属の成分を規定し、さらに溶接後、(A1変態点+100℃)〜1000℃に温度に再加熱する技術が開示されている。しかし、特許文献4の技術は、2相域加熱部に生成した島状マルテンサイトを微細化するために、もう一度、高温に加熱する必要があり、生産性を著しく阻害する。また、この技術は、比較的板厚の薄い鋼板を使用する鋼管に適用する技術であり、海洋構進物などの大型鋼構造物には適用することは難しい。 Further, Patent Document 4, in order to suppress the decrease in toughness due to island martensite two-phase region heating unit defines a base material and components of the weld metal, after further welding, (A 1 transformation point + 100 ° C.) ~ A technique for reheating to 1000 ° C. is disclosed. However, in the technique of Patent Document 4, in order to refine the island-shaped martensite generated in the two-phase zone heating unit, it is necessary to heat it to a high temperature once again, which significantly impedes productivity. Moreover, this technique is a technique applied to a steel pipe using a steel plate having a relatively thin plate thickness, and is difficult to apply to a large steel structure such as a marine structure.
さらに、上記従来技術には、以下のような解決すべき課題が残されている。
例えば、Ti酸化物等を利用する技術では、酸化物等を鋼中に均一に微細分散させることが難しいという問題がある。さらに、近年では、構造物が大型化するのに伴い、使用される鋼材の高強度化や厚肉化が要求されており、それらの要求に応えるためには、従来技術以上に合金元素を添加することが必要となる。しかし、合金元素の過度の添加は、溶接熱影響部の靭性を低下させるため、好ましくないという問題もある。
Furthermore, the following problems to be solved remain in the conventional technology.
For example, a technique using Ti oxide or the like has a problem that it is difficult to uniformly disperse the oxide or the like in steel. Furthermore, in recent years, with the increase in size of structures, it has been required to increase the strength and thickness of steel materials used. In order to meet these requirements, alloying elements have been added over conventional technologies. It is necessary to do. However, excessive addition of alloy elements also has the problem that it is not preferable because it reduces the toughness of the weld heat affected zone.
本発明は、従来技術が抱える上記問題点に鑑みてなされたものであり、その目的は、板厚30mm以上、降伏強度が630MPa以上で、母材の強度・靭性に優れるとともに、溶接熱影響部の靭性にも優れる高張力鋼板とその有利な製造方法を提案することにある。 The present invention has been made in view of the above-mentioned problems of the prior art, and its objectives are a plate thickness of 30 mm or more, a yield strength of 630 MPa or more, excellent base material strength and toughness, and a weld heat affected zone. The present invention proposes a high-tensile steel sheet having excellent toughness and an advantageous manufacturing method thereof.
発明者らは、厚肉かつ高強度が求められる鋼板の母材強度・靭性を向上するだけでなく、溶接熱影響部の靭性をも改善する方法について、鋭意検討を重ねた。その結果、溶接熱影響部のボンド部近傍における靭性低下は、多層溶接時に2相域に加熱される部分に形成される島状マルテンサイトを含む脆化組織の生成に起因するものであることを確認するとともに、上記島状マルテンサイトに対する対策が、従来技術ではまだ不十分であることを知見した。 The inventors have made extensive studies on a method for improving not only the base material strength and toughness of a steel sheet that is required to be thick and having high strength, but also the toughness of the weld heat affected zone. As a result, the toughness reduction in the vicinity of the bond portion of the weld heat affected zone is due to the formation of an embrittled structure including island martensite formed in the portion heated to the two-phase region during multilayer welding. While confirming, it was found that the countermeasures against the island-shaped martensite are still insufficient with the prior art.
そこで、発明者らは、上記脆化組織を改善する方法についてさらに検討した結果、従来技術のように単にCを低減するだけでは不十分であり、さらに、生成する島状マルテンサイトの大きさ(面積)を小さくすると共に、島状マルテンサイトの硬さを低減してマトリックス組織との硬度差を小さくしてやる必要があること、そして、その達成手段としては、母材成分中のMn,NiおよびCrを適正量添加し、Cを低減してやることが有効であることを見出し、本発明を完成させた。 Thus, as a result of further study on the method for improving the above-mentioned embrittlement structure, the inventors simply do not reduce C as in the prior art, and further, the size of island martensite to be generated ( Area) and the hardness of the island martensite must be reduced to reduce the hardness difference from the matrix structure, and as means for achieving this, Mn, Ni and Cr in the base material component It was found that it is effective to add a proper amount of C and reduce C, and the present invention was completed.
すなわち、本発明は、C:0.039〜0.2mass%、Si:0.05〜0.3mass%、Mn:0.5〜5mass%、P:0.015mass%以下、S:0.005mass%以下、Cr:3mass%以下、Ni:5mass%以下、Ti:0.005〜0.02mass%、Al:0.004mass%以下、N:0.007mass%以下、B:0.0003〜0.003mass%を含有し、かつ、Mn,Ni,CrおよびCが下記(1)式;
Mn+Ni+Cr−12.5×C≧2.6(mass%) ・・・(1)
を満たして含有し、残部がFeおよび不可避的不純物からなる成分組成を有し、入熱量が80kJ/cm以下の多層溶接を施したときの溶接熱影響部に形成される島状マルテンサイトの平均面積が3μm2以下である板厚が30mm以上の高張力鋼板である。ここで、上記式中の各元素記号は、その元素の含有量(mass%)を示す。
That is, the present invention is C: 0.039 to 0.2 mass%, Si: 0.05 to 0.3 mass%, Mn: 0.5 to 5 mass%, P: 0.015 mass% or less, S: 0.005 mass. %: Cr: 3 mass% or less, Ni: 5 mass% or less, Ti: 0.005-0.02 mass%, Al: 0.004 mass% or less, N: 0.007 mass% or less, B: 0.0003-0. 003 mass%, and Mn, Ni, Cr and C are represented by the following formula (1):
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
Of island martensite formed in the weld heat affected zone when multi-layer welding with a heat input of 80 kJ / cm or less is applied. This is a high-tensile steel plate having an area of 3 μm 2 or less and a thickness of 30 mm or more . Here, each element symbol in the above formula indicates the content (mass%) of the element.
本発明の高張力鋼板は、上記成分組成に加えてさらに、Cu:0.5mass%以下、Mo:1mass%以下、V:0.2mass%以下およびNb:0.1mass%以下のうちから選ばれる1種または2種以上を含有することを特徴とする。 The high-tensile steel sheet of the present invention is further selected from Cu: 0.5 mass% or less, Mo: 1 mass% or less, V: 0.2 mass% or less, and Nb: 0.1 mass% or less in addition to the above component composition. 1 type or 2 types or more are contained, It is characterized by the above-mentioned.
また、本発明の高張力鋼板は、上記成分組成に加えてさらに、Ca:0.0005〜0.003mass%およびREM:0.0003〜0.003mass%のうちから選ばれる1種または2種を含有することを特徴とする。 In addition to the above component composition, the high-tensile steel plate of the present invention further includes one or two selected from Ca: 0.0005 to 0.003 mass% and REM: 0.0003 to 0.003 mass%. It is characterized by containing.
また、本発明は、上記いずれかに記載の成分組成を有する鋼素材を、Ac3変態点〜1200℃の温度に加熱後、累積圧下率50%以上の熱間加工を施し、次いで、そのままAr3変態点以上の温度から板厚中心部の温度が350℃以下になるまで1℃/s以上で冷却して鋼板全体を焼き入れし、あるいは、放冷してからAc3変態点〜1050℃の温度に再加熱した後に板厚中心部の温度が350℃以下になるまで1℃/s以上で冷却して鋼板全体を焼き入れし、その後、450〜650℃の温度で焼戻処理を施す高張力鋼板の製造方法を提案する。 In addition, the present invention is to heat a steel material having any of the above-described component compositions to a temperature of Ac 3 transformation point to 1200 ° C., then subject to hot working with a cumulative rolling reduction of 50% or more, and then continue with Ar as it is. The steel plate is cooled at a rate of 1 ° C./s or higher until the temperature at the center of the plate thickness is 350 ° C. or lower from the temperature of the third transformation point or higher, or the entire steel plate is quenched , or after standing to cool, the Ac 3 transformation point to 1050 ° C. After being reheated to a temperature of 1 ° C., the whole steel sheet is quenched by cooling at 1 ° C./s or more until the temperature at the center of the plate thickness becomes 350 ° C. or less, and then tempered at a temperature of 450 to 650 ° C. A method for manufacturing high-tensile steel sheets is proposed.
本発明によれば、板厚が30mm以上で、降伏強度が630MPa以上の高強度を有し、母材の靭性にも優れると共に、多層溶接した熱影響部の靭性にも優れる高張力鋼板を安定して製造することが可能となる。 According to the present invention, a high-strength steel sheet having a thickness of 30 mm or more, a yield strength of 630 MPa or more, excellent base material toughness, and excellent heat-affected zone toughness in multi-layer welding can be stabilized. And can be manufactured.
上述したように、発明者らは、厚肉かつ高強度が求められる鋼板(母材)の強度・靭性を向上するとともに、溶接熱影響部の靭性をも改善する方法について、検討を重ねた結果、溶接熱影響部のボンド部近傍における靭性低下は、多層溶接時の2相域加熱部に形成される島状マルテンサイトを含む脆化組織の生成に起因することを確認した。そして、上記島状マルテンサイトによる溶接熱影響部のボンド部近傍の靭性低下を改善するには、従来技術のように単にCを低減するだけでは不十分であり、さらに、生成する島状マルテンサイトの大きさを、平均面積で3μm2以下に小さくすると共に、島状マルテンサイトの硬さを低減してマトリックス組織との硬度差を小さくしてやる必要があること、そして、その達成手段としては、所定の成分組成を満足した上で、母材成分中のC,Mn,NiおよびCrを下記(1)式;
Mn+Ni+Cr−12.5×C≧2.6(mass%) ・・・(1)
(上記式中の各元素記号は、その元素の含有量(mass%)を示す。)
を満たして含有させる必要があることを見出した。
As described above, the inventors have repeatedly studied a method for improving the strength and toughness of a steel plate (base material) that is required to be thick and having high strength, and also improving the toughness of the weld heat affected zone. It was confirmed that the decrease in toughness in the vicinity of the bond portion of the weld heat affected zone was caused by the formation of an embrittled structure including island martensite formed in the two-phase zone heating zone during multi-layer welding. And, in order to improve the toughness reduction in the vicinity of the bond portion of the weld heat affected zone due to the island-shaped martensite, it is not sufficient to simply reduce C as in the prior art. The average area must be reduced to 3 μm 2 or less, and the hardness of the island martensite must be reduced to reduce the hardness difference from the matrix structure. In addition, the C, Mn, Ni and Cr in the base material component are represented by the following formula (1):
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
(Each element symbol in the above formula indicates the content (mass%) of the element.)
It was found that it is necessary to satisfy and contain.
ここで、上記(1)式の意味するところは、以下のとおりである。
MnおよびNiは、オーステナイト安定化元素であるため、これらの元素の含有量を高めることによって、オーステナイト中に固溶するCの濃度上昇を抑制することができる。さらに、炭化物安定化元素であるCrを添加し、析出した炭化物の再溶解を抑制することによっても、オーステナイト中に固溶するC濃度をより低減させることができる。これらの固溶C濃度の低下効果によって、溶接熱影響部に生成する島状マルテンサイトの一つ一つを微細化し、平均面積で3μm2以下とすることができると共に、島状マルテンサイトの硬さを低下し、マトリックス組織との硬度差を小さくすることができる。その結果、島状マルテンサイトが破壊の起点になり難くなり、溶接部の靭性を顕著に向上することが可能となる。
Here, the meaning of the above formula (1) is as follows.
Since Mn and Ni are austenite stabilizing elements, increasing the content of these elements can suppress an increase in the concentration of C that dissolves in austenite. Furthermore, the concentration of C dissolved in austenite can be further reduced by adding Cr, which is a carbide stabilizing element, and suppressing re-dissolution of the precipitated carbide. Due to the effect of reducing the solute C concentration, each of the island martensites generated in the weld heat affected zone can be refined to an average area of 3 μm 2 or less. The hardness can be reduced, and the hardness difference from the matrix structure can be reduced. As a result, island martensite is less likely to be a starting point of fracture, and the toughness of the welded portion can be significantly improved.
なお、本発明における溶接熱影響部に形成される島状マルテンサイトの平均面積とは、溶接熱影響部の断面において観察される個々の島状マルテンサイトの面積の平均値のことを意味する。また、その測定方法としては、例えば、溶接熱影響部の断面を2段エッチングして島状マルテンサイトを現出させた後、2相域に加熱されるボンド部近傍を、走査型電子顕微鏡(SEM)を用いて倍率3000倍で10視野撮影し、画像解析して個々の島状マルテンサイトの面積を測定し、それを平均することで求めることができる。 In addition, the average area of the island martensite formed in the welding heat affected zone in the present invention means the average value of the area of individual island martensite observed in the cross section of the weld heat affected zone. In addition, as a measuring method, for example, the cross section of the weld heat affected zone is etched in two steps to reveal island martensite, and then the vicinity of the bond portion heated in the two-phase region is scanned with a scanning electron microscope ( It can be obtained by taking 10 fields of view using a SEM) at a magnification of 3000 times, analyzing the image, measuring the area of each island-like martensite, and averaging them.
次に、本発明の鋼板が有すべき成分組成について具体的に説明する。
C:0.005〜0.2mass%
Cは、構造用鋼としての本発明の鋼板に求められる強度(降伏強度≧630MPa)を確保するために必要不可欠の元素である。Cが0.005mass%未満では、上記必要な強度を確保することができなかったり、他の合金元素の多量添加が必要となったりするため原料コストの上昇を招く。一方、0.2mass%を超えて添加すると、溶接熱影響部に生成する島状マルテンサイトの生成量が増加し、個々の島状マルテンサイトが粗大化しやすくなり、さらに、島状マルテンサイト中のC濃度も高くなって硬さが上昇するため、溶接熱影響部の靭性が大きく低下してしまう。よって、Cは0.005〜0.2mass%の範囲とする。好ましくは0.01〜0.15mass%の範囲である。
Next, the component composition that the steel sheet of the present invention should have will be specifically described.
C: 0.005-0.2 mass%
C is an indispensable element for ensuring the strength (yield strength ≧ 630 MPa) required for the steel plate of the present invention as structural steel. If C is less than 0.005 mass%, the required strength cannot be ensured, or a large amount of other alloy elements must be added, leading to an increase in raw material costs. On the other hand, if it exceeds 0.2 mass%, the amount of island martensite generated in the weld heat affected zone increases, and the individual island martensite is likely to be coarsened. Further, in the island martensite Since C density | concentration also becomes high and hardness rises, the toughness of a welding heat affected zone will fall large. Therefore, C is in the range of 0.005 to 0.2 mass%. Preferably it is the range of 0.01-0.15 mass%.
Si:0.05〜0.3mass%
Siは、鋼の脱酸材として添加される必須の元素であり、本発明では0.05mass%以上添加する必要がある。しかし、0.3mass%を超えて添加すると、母材靭性のみならず、島状マルテンサイトの生成を助長して溶接熱影響部の靭性をも低下させる。よって、本発明では、Siは0.3mass%以下とする。
Si: 0.05-0.3 mass%
Si is an essential element added as a deoxidizer for steel, and in the present invention, it is necessary to add 0.05 mass% or more. However, if added over 0.3 mass%, not only the base material toughness but also the formation of island martensite is promoted and the toughness of the weld heat affected zone is also lowered. Therefore, in this invention, Si shall be 0.3 mass% or less.
Mn:0.5〜5mass%
Mnは、脱酸材として、また、母材強度を確保する観点から0.5mass%以上添加する必要がある。一方、5mass%を超えて添加すると、焼入性が過剰に高まり、溶接熱影響部の靭性を低下させる。よって、Mnは、0.5〜5mass%の範囲とする。好ましくは、0.5〜2mass%の範囲である。
Mn: 0.5-5 mass%
Mn needs to be added in an amount of 0.5 mass% or more as a deoxidizing material and from the viewpoint of securing the strength of the base material. On the other hand, if added over 5 mass%, the hardenability is excessively increased and the toughness of the weld heat affected zone is lowered. Therefore, Mn is set to a range of 0.5 to 5 mass%. Preferably, it is the range of 0.5-2 mass%.
P:0.015mass%以下
Pは、靭性を低下させる有害な元素である。特に、0.015mass%を超えて含有すると、母材および溶接熱影響部の靭性を低下させる。よって、本発明では、Pは0.015mass%以下に制限する。
P: 0.015 mass% or less P is a harmful element that lowers toughness. In particular, when it contains exceeding 0.015 mass%, the toughness of a base material and a welding heat affected zone will be reduced. Therefore, in the present invention, P is limited to 0.015 mass% or less.
S:0.005mass%以下
Sも、靭性を低下させる有害な元素である。特に、0.005mass%を超えて含有すると、母材および溶接熱影響部の靭性を低下させる。よって、本発明では、Sは0.005mass%以下に制限する。
S: 0.005 mass% or less S is also a harmful element that reduces toughness. In particular, when it contains exceeding 0.005 mass%, the toughness of a base material and a welding heat affected zone will be reduced. Therefore, in the present invention, S is limited to 0.005 mass% or less.
Cr:3mass%以下
Crは、鋼(母材)の高強度化に有効な元素であるが、過剰に含有させると、却って靭性を低下させるので、本発明では上限を3mass%とする。好ましくは0.1〜2.7mass%、より好ましくは0.4〜2.5mass%の範囲である。
Cr: 3 mass% or less Cr is an element effective for increasing the strength of steel (base material). However, if excessively contained, the toughness is lowered, and therefore, the upper limit is set to 3 mass% in the present invention. Preferably it is 0.1-2.7 mass%, More preferably, it is the range of 0.4-2.5 mass%.
Ni:5mass%以下
Niは、鋼(母材)の強度および溶接熱影響部の靭性を向上するのに有効な元素である。しかし、Niは高価な元素であるため、上限を5mass%とする。好ましくは0.5〜5mass%、より好ましくは0.7〜3mass%の範囲である。
Ni: 5 mass% or less Ni is an element effective for improving the strength of steel (base material) and the toughness of the weld heat affected zone. However, since Ni is an expensive element, the upper limit is set to 5 mass%. Preferably it is 0.5-5 mass%, More preferably, it is the range of 0.7-3 mass%.
Ti:0.005〜0.02mass%
Tiは、鋼中で窒化物を形成し、固溶窒素量を低減し、BNの析出を抑制するので、焼き入れに必要なBを確保するのに有効な元素でもある。さらに、Tiの窒化物は、オーステナイト温度域でも安定な析出物であり、溶接熱影響部のオーステナイトの粗大化を効果的に抑制する。斯かる効果は0.005mass%以上含有させることにより得られる。しかし、0.02mass%を超えて添加すると、析出した窒化物が粗大化し、母材および溶接熱影響部の靭性を低下させるので、上限は0.02mass%とする必要がある。
Ti: 0.005-0.02 mass%
Ti forms a nitride in the steel, reduces the amount of dissolved nitrogen, and suppresses the precipitation of BN, and is therefore an effective element for securing B necessary for quenching. Furthermore, Ti nitride is a stable precipitate even in the austenite temperature range, and effectively suppresses austenite coarsening in the weld heat affected zone. Such an effect can be obtained by adding 0.005 mass% or more. However, if added over 0.02 mass%, the deposited nitride is coarsened and the toughness of the base metal and the weld heat affected zone is lowered, so the upper limit needs to be 0.02 mass%.
Al:0.004mass%以下
本発明では、高強度鋼板を製造する鋼素材として、Al脱酸鋼ではなく、Siおよび/またはMn脱酸鋼を用いる。ただし、予備処理としてSiやMn脱酸に先立ち、Alを添加して予備脱酸を行ってもよい。ただし、Alを脱酸材として添加してする場合には、溶解中に残留するAlは0.004mass%以下とすることが必要である。Alが0.004mass%を超えて含有すると、溶接熱影響部における島状マルテンサイトの生成を助長して靭性を低下させるためである。
Al: 0.004 mass% or less In the present invention, Si and / or Mn deoxidized steel is used as a steel material for producing a high-strength steel sheet, not Al deoxidized steel. However, as a preliminary treatment, prior to Si or Mn deoxidation, Al may be added to perform preliminary deoxidation. However, when Al is added as a deoxidizing material, the Al remaining during dissolution needs to be 0.004 mass% or less. This is because when Al exceeds 0.004 mass%, the formation of island martensite in the weld heat-affected zone is promoted to reduce toughness.
N:0.007mass%以下
Nは、鋼(母材)中に過剰に固溶すると、母材の靭性を低下させる。また、過剰なNの含有は、溶接熱影響部においても、粗大な窒化物や炭窒化物を形成して靭性を低下させる。よって、本発明では、Nを0.007mass%以下に制限する。
N: 0.007 mass% or less N, when excessively dissolved in steel (base material), lowers the toughness of the base material. Further, excessive N content also forms coarse nitrides and carbonitrides in the weld heat affected zone, thereby reducing toughness. Therefore, in the present invention, N is limited to 0.007 mass% or less.
B:0.0003〜0.003mass%
Bは、オーステナイト粒界に偏析して粒界からのフェライト変態を抑制し、ベイナイト変態やマルテンサイト変態を促進する効果があるので、高強度化、高靱性化には有効な元素である。この効果を得るためには0.0003mass%以上の含有が必要である。しかし、0.003mass%を超えて添加すると、炭窒化物となって析出し、焼入性を低下させたり、靭性を低下させたりするようになる。よって、本発明では、Bは0.0003〜0.003mass%の範囲とする。好ましくは0.0005〜0.0020mass%の範囲である。
B: 0.0003 to 0.003 mass%
B segregates at the austenite grain boundaries and suppresses the ferrite transformation from the grain boundaries, and has the effect of promoting the bainite transformation and martensite transformation. Therefore, B is an effective element for increasing the strength and toughness. In order to acquire this effect, 0.0003 mass% or more needs to be contained. However, when added over 0.003 mass%, it becomes carbonitride and precipitates, resulting in a decrease in hardenability and a decrease in toughness. Therefore, in the present invention, B is in the range of 0.0003 to 0.003 mass%. Preferably it is the range of 0.0005-0.0020 mass%.
本発明の高張力鋼板は、上記必須成分に加えてさらに、強度・靭性を高める目的で、Cu,Mo,VおよびNbの中から選ばれる1種または2種以上を下記の範囲で含有させることができる。
Cu:0.5mass%以下
Cuは、低温靭性を損なうことなく鋼の強度を高めることができる元素である。しかし、0.5mass%を超えて含有させると、熱間加工時に鋼板表面に割れを生じるようになるので、Cuを添加する場合には上限は0.5mass%とするのが好ましい。
In addition to the above essential components, the high-tensile steel sheet of the present invention further contains one or more selected from Cu, Mo, V and Nb in the following ranges for the purpose of increasing strength and toughness. Can do.
Cu: 0.5 mass% or less Cu is an element that can increase the strength of steel without impairing low-temperature toughness. However, if the content exceeds 0.5 mass%, the steel sheet surface is cracked during hot working. Therefore, when Cu is added, the upper limit is preferably 0.5 mass%.
Mo:1mass%以下
Moは、母材を高強度化するのに有効な元素である。しかし、1mass%を超えて含有させると、炭化物の析出により硬度が上昇し、靭性を低下させる。よって、Moを含有させる場合には1mass%以下とするのが好ましい。
Mo: 1 mass% or less Mo is an element effective for increasing the strength of the base material. However, if the content exceeds 1 mass%, the hardness increases due to precipitation of carbides, and the toughness decreases. Therefore, when it contains Mo, it is preferable to set it as 1 mass% or less.
V:0.2mass%以下
Vは、母材の強度・靭性の向上に有効な元素であり、また、VNとして析出し、固溶Nを低減するのにも有効な元素である。しかし、0.2mass%を超えて含有させると、硬質なVCの析出により靭性が低下するようになるので、Vを含有させる場合には上限は0.2mass%とするのが好ましい。より好ましくは0.1mass%以下である。
V: 0.2 mass% or less V is an element effective for improving the strength and toughness of the base material, and is also an element effective for precipitation as VN and for reducing solid solution N. However, if the content exceeds 0.2 mass%, the toughness decreases due to precipitation of hard VC. Therefore, when V is contained, the upper limit is preferably 0.2 mass%. More preferably, it is 0.1 mass% or less.
Nb:0.1mass%以下
Nbは、鋼の強度を高めるのに有効な元素である。しかし、0.1mass%を超えて含有させると、溶接熱影響部の靭性を低下させるので、Nbを含有させる場合には上限を0.1mass%とするのが好ましい。さらに好ましくは、0.010〜0.020%である。
Nb: 0.1 mass% or less Nb is an element effective for increasing the strength of steel. However, if the content exceeds 0.1 mass%, the toughness of the weld heat-affected zone is lowered. Therefore, when Nb is contained, the upper limit is preferably set to 0.1 mass%. More preferably, it is 0.010 to 0.020%.
本発明の高張力鋼板は、上記成分に加えてさらに、機械的特性を改善する目的で、CaおよびREMのうちから選ばれる1種または2種を、Ca:0.0005〜0.003mass%、REM:0.0005〜0.003mass%の範囲で含有させることができる。
CaおよびREMは、有害なOおよびNを酸化物および硫化物として固定し、鋼の機械的特性を改善する効果があるため、それぞれ0.0005mass%以上含有させることができる。しかし、いずれも0.003mass%を超えて含有させても、その効果が飽和するため、上限は0.003mass%とするのが好ましい。なお、上記REM(レア・アース・メタル)とは、La,Ceをはじめとする希土類元素のことをいう。
In addition to the above components, the high-tensile steel sheet of the present invention further includes one or two selected from Ca and REM for the purpose of improving mechanical properties, Ca: 0.0005 to 0.003 mass%, REM: It can be contained in the range of 0.0005 to 0.003 mass%.
Since Ca and REM have the effect of fixing harmful O and N as oxides and sulfides and improving the mechanical properties of steel, they can be contained in amounts of 0.0005 mass% or more, respectively. However, even if both are contained exceeding 0.003 mass%, the effect is saturated, so the upper limit is preferably set to 0.003 mass%. The REM (rare earth metal) refers to rare earth elements including La and Ce.
本発明の高張力鋼板は、上記成分以外の残部は、Feおよび不可避的不純物からなる。ただし、本発明の作用効果を害しない範囲であれば、他の成分を含有していてもよい。 In the high-tensile steel sheet of the present invention, the balance other than the above components is composed of Fe and inevitable impurities. However, other components may be contained as long as the effects of the present invention are not impaired.
次に、本発明の高張力鋼板の製造方法について説明する。
鋼素材
本発明の高張力鋼板の素材となる鋼素材(スラブ、ビレット等)は、上記した成分組成の鋼を、例えば、転炉、電気炉、真空溶解炉等の通常の製錬プロセスで溶製した後、連続鋳造法あるいは造塊−分塊圧延法等、通常公知の方法を用いて製造することができ、特に制限はない。
Next, the manufacturing method of the high-tensile steel plate of this invention is demonstrated.
Steel material The steel material (slab, billet, etc.) that is the material of the high-strength steel sheet of the present invention is obtained by melting steel having the above-described composition in a normal smelting process such as a converter, electric furnace, vacuum melting furnace, etc. After the production, it can be produced by a generally known method such as a continuous casting method or an ingot-bundling rolling method, and there is no particular limitation.
熱間加工(熱間圧延および/または熱間鍛造)
次いで、本発明では、上記鋼素材をAc3変態点〜1200℃の温度に加熱した後、累積圧下率が50%以上の熱間加工し、高張力鋼板とする。ここで、上記の熱間加工とは、熱間圧延、熱間鍛造あるいは熱間鍛造と熱間圧延の組み合せのいずれかの加工方法を意味する。
上記鋼素材の加熱温度を、Ac3変態点以上とする理由は、Ac3変態点未満の温度では、オーステナイト単相域にならないため、鋼組織の均一性が悪く、製造安定性が著しく低下するからである。また、1200℃以下とする理由は、1200℃を超えて加熱しても、熱エネルギーのロスとなるだけだからである。ここで、上記加熱温度とは、鋼素材の厚さ中心部の温度を指すものとする。厚さ中心部の温度は、鋼素材の厚さ、表面温度および冷却条件などから、シミュレーション計算などで求めることができる。例えば、差分法を用いて厚さ方向の温度分布を計算することにより、厚さ中心部の温度を求めることができる。また、熱間加工における累積圧下率を50%以上とする理由は、鋼素材の板厚中心部まで十分な加工を加えて、鋼組織を微細化するためである。
ここで、上記Ac3変態点は、実測して求めることができるが、下記(2)式から算出してもよい。
記
Ac3変態点(℃)=937.2−476.5×C+56×Si−19.7×Mn−16.3×Cu−26.6×Ni−4.9×Cr+38.1×Mo+124.8×V+136.3×Ti−19.1×Nb+198.4×Al+3315×B
・・・(2)
(上記式中の各元素記号は、その元素の含有量(mass%)を示す。)
なお、熱間加工の終了温度は、鋼板組織の均一性を確保する観点から、Ar3変態点以上とするのが好ましい。
Hot working (hot rolling and / or hot forging)
Next, in the present invention, the steel material is heated to a temperature of Ac 3 transformation point to 1200 ° C. and then hot-worked with a cumulative rolling reduction of 50% or more to obtain a high-tensile steel plate. Here, the above hot working means any one of hot rolling, hot forging, or a combination of hot forging and hot rolling.
The heating temperature of the steel material, the reason for the Ac 3 transformation point or higher at temperatures below Ac 3 transformation point, and since they are not in the austenite single-phase region, the uniformity of the steel structure is poor, manufacturing stability is remarkably lowered Because. Moreover, the reason for setting it as 1200 degrees C or less is because even if it heats exceeding 1200 degreeC, it will only be a loss of thermal energy. Here, the said heating temperature shall point out the temperature of the thickness center part of a steel raw material. The temperature at the center of the thickness can be obtained by simulation calculation or the like from the thickness of the steel material, the surface temperature, the cooling conditions, and the like. For example, the temperature at the center of the thickness can be obtained by calculating the temperature distribution in the thickness direction using the difference method. The reason why the cumulative reduction ratio in hot working is 50% or more is to refine the steel structure by applying sufficient working to the center of the plate thickness of the steel material.
Here, the Ac 3 transformation point can be obtained by actual measurement, but may be calculated from the following equation (2).
Notation Ac 3 transformation point (° C.) = 937.2−476.5 × C + 56 × Si−19.7 × Mn−16.3 × Cu−26.6 × Ni−4.9 × Cr + 38.1 × Mo + 1244.8 * V + 136.3 * Ti-19.1 * Nb + 198.4 * Al + 3315 * B
... (2)
(Each element symbol in the above formula indicates the content (mass%) of the element.)
Incidentally, the end temperature of the hot working, in order to ensure the uniformity of the steel sheet structure, preferably the Ar 3 transformation point or more.
熱間加工後の冷却、あるいは、熱間加工後の再加熱とその後の冷却
上記のように熱間加工(熱間圧延および/または熱間鍛造)した鋼板は、次いで、そのままAr3変態点以上の温度から板厚中心部が350℃以下になるまで急冷するか、あるいは、放冷してからAc3変態点〜1050℃の温度に再加熱した後に板厚中心部が350℃以下になるまで急冷する、のいずれかの方法で焼入れする。
ここで、熱間加工後にそのまま急冷する場合に、急冷開始温度をAr3変態点以上とする理由は、冷却開始前の組織をオーステナイト単相組織とするためである。なお、この急冷開始温度は、鋼板表面温度とする。
一方、熱間加工した鋼板を再加熱した後に急冷する場合に、再加熱温度を1050℃以下とする理由は、これを超える温度ではオーステナイト粒が粗大化し、母材の靭性が低下するためである。また、再加熱温度をAc3変態点以上の温度とする理由は、焼入れ前の鋼板をオーステナイト単相組織とすることにより、焼入れ後、および、焼戻し後の鋼板組織・材質を均質化するためである。これにより、所望の強度・靱性を達成することができる。ここで、上記再加熱温度とは、鋼板の板厚中心部の温度を指すものとする。板厚中心部の温度は、板厚、表面温度および冷却条件などから、シミュレーション計算などで求めることができる。たとえば、差分法を用い、板厚方向の温度分布を計算することにより、板厚中心部の温度を求めることができる。
なお、上記Ar3変態点は、実測してもよいが、下記(3)式から求めてもよい。
記
Ar3変態点(℃)=910−273×C−74×Mn−16×Cr−9×Mo−5×Cu ・・・(3)
(上記式中の各元素記号は、その元素の含有量(mass%)を示す。)
Cooling after hot working, or reheating after hot working and subsequent cooling The steel sheet that has been hot worked (hot rolled and / or hot forged) as described above then has an Ar 3 transformation point or higher as it is. From this temperature, the sheet thickness center is rapidly cooled to 350 ° C. or lower, or after standing to cool to a temperature of Ac 3 transformation point to 1050 ° C., until the thickness center reaches 350 ° C. or lower. Quench by either method of rapid cooling.
Here, when quenching as it is after hot working, the reason for setting the rapid cooling start temperature to the Ar 3 transformation point or more is to make the structure before the cooling start an austenite single phase structure. The rapid cooling start temperature is the steel sheet surface temperature.
On the other hand, when the hot-worked steel sheet is rapidly cooled after being reheated, the reason for setting the reheating temperature to 1050 ° C. or less is that the austenite grains become coarse at temperatures exceeding this, and the toughness of the base material is reduced. . The reason why the reheating temperature is set to the Ac 3 transformation point or higher is to make the steel sheet structure and material after quenching and tempering uniform by making the steel sheet before quenching an austenite single phase structure. is there. Thereby, desired strength and toughness can be achieved. Here, the said reheating temperature shall point out the temperature of the plate | board thickness center part of a steel plate. The temperature at the center of the plate thickness can be obtained by simulation calculation or the like from the plate thickness, surface temperature, cooling conditions, and the like. For example, the temperature at the center of the plate thickness can be obtained by calculating the temperature distribution in the plate thickness direction using the difference method.
The Ar 3 transformation point may be actually measured, but may be obtained from the following equation (3).
Ar 3 transformation point (° C.) = 910-273 × C-74 × Mn-16 × Cr-9 × Mo-5 × Cu (3)
(Each element symbol in the above formula indicates the content (mass%) of the element.)
また、Ar3変態点以上の温度から急冷するときの冷却停止温度を、板厚中心部で350℃以下とするのは、鋼板全体を焼入れするためである。これにより、板厚全体にわたってベイナイトまたはマルテンサイト変態が確実に開始するので、後述する焼戻し処理まで完了した時点において、板厚全体にわたって、焼戻しベイナイト組織または/およびマルテンサイト組織とすることができる。 The reason why the cooling stop temperature when quenching from the temperature above the Ar 3 transformation point is 350 ° C. or less at the center of the plate thickness is to quench the entire steel plate. Thereby, since the bainite or martensite transformation is surely started over the entire plate thickness, the tempered bainite structure and / or the martensite structure can be formed over the entire plate thickness when the tempering process described later is completed.
したがって、Ar3変態点以上の温度からの冷却速度も、連続冷却変態図(CCT曲線)のFs点(フェライト変態開始温度)を通過せずに、Bs点(ベイナイト変態開始温度)またはMs点(マルテンサイト変態開始温度)を通過できる速度であればよく、特に限定されないが、概ね1℃/秒以上とするのが好ましい。また、上記急冷する方法は水冷を採用することができるが、上記冷却速度が確保できる方法であれば水冷に限定されるものではなく、ガス冷却でもよい。 Therefore, the cooling rate from the temperature above the Ar 3 transformation point does not pass through the Fs point (ferrite transformation start temperature) of the continuous cooling transformation diagram (CCT curve), but the Bs point (bainite transformation start temperature) or Ms point ( There is no particular limitation as long as it can pass through the martensite transformation start temperature), but it is preferably about 1 ° C./second or more. Moreover, although the water cooling can be employ | adopted for the method of the said rapid cooling, if it is a method which can ensure the said cooling rate, it will not be limited to water cooling, Gas cooling may be sufficient.
上記焼入後の鋼板は、ベイナイト組織および/またはマルテンサイト組織となる。
なお、工業的には、鋼の強靭化を目的として、焼入れを繰り返すことがあり、本発明においても、繰り返し焼入れしてもよい。ただし、最終焼入れの際には、上記冷却条件を満たすことが必要である。
The steel sheet after quenching has a bainite structure and / or a martensite structure.
Industrially, quenching may be repeated for the purpose of toughening steel, and in the present invention, quenching may be repeated. However, at the time of final quenching, it is necessary to satisfy the above cooling conditions.
焼戻処理
熱間加工後、上記のいずれかの方法で急冷し、焼入れした鋼板は、その後、再加熱し、450〜650℃の温度で焼戻処理を施す必要がある。焼戻温度が450℃未満では、焼入れに伴う残留応力の除去効果が十分ではなく、一方、650℃を超えると、鋼中に種々の炭窒化物が析出するとともに、変態で得られた微細組織が消失し、強度・靭性が大幅に低下してしまうためである。上記焼戻処理により、鋼板組織は、焼戻しベイナイト組織および/または焼戻しマルテンサイト組織となる。なお、その他に、フェライト組織、パーライト組織、残留オーステナイト組織等が存在することもあるが、それらの合計が面積率で5%以下であれば、本発明の作用効果に影響はない。上記のように、本発明の高張力鋼板は、焼入れ後、焼戻処理を施すことが重要であり、これらの熱処理を施すことによって、母材の強度および靭性に優れる鋼板を製造することができる。
Tempering treatment After hot working, the steel sheet quenched and quenched by any of the above methods must be reheated and tempered at a temperature of 450 to 650 ° C. When the tempering temperature is less than 450 ° C, the effect of removing the residual stress accompanying quenching is not sufficient. On the other hand, when the temperature exceeds 650 ° C, various carbonitrides are precipitated in the steel and the microstructure obtained by transformation is obtained. This disappears, and the strength and toughness are greatly reduced. By the tempering treatment, the steel sheet structure becomes a tempered bainite structure and / or a tempered martensite structure. In addition, a ferrite structure, a pearlite structure, a retained austenite structure, and the like may be present. However, if the total of these is 5% or less in area ratio, there is no influence on the function and effect of the present invention. As described above, it is important that the high-tensile steel sheet of the present invention is tempered after quenching, and by performing these heat treatments, it is possible to produce a steel sheet that is excellent in the strength and toughness of the base material. .
表1に示したNo.1〜29の鋼を溶製し、鋼素材(スラブ)とした後、表2に示した条件で、加熱し、累積圧下率が50〜90%の熱間圧延を施して板厚が50〜150mmの鋼板とし、その後、その厚鋼板をそのまま急冷して焼入れし、あるいは放冷した後に再加熱してから急冷して焼入れし、その後、焼戻処理を施して、鋼板No.1〜33の製品鋼板を製造した。斯くして得られた鋼板を下記の試験に供した。
<引張試験>
各鋼板の板厚1/4の位置から、圧延方向を引張方向とするJIS4号引張試験片を採取し、引張試験を実施して降伏強度および引張強さ(TS)を測定した。
<衝撃試験>
各鋼板の板厚1/4の位置から、圧延方向を長手方向とするVノッチシャルピー衝撃試験片を3本ずつ採取し、各試験片について−60℃でシャルピー衝撃試験を行い、吸収エネルギー(vE−60)を測定し、それらの平均値を求めた。
<溶接後衝撃試験>
各鋼板から採取した2枚の溶接用試験片に、X開先(開先角度45°)加工を施した後、入熱50kJ/cmのサブマージアーク溶接を行い、多層溶接継手を作製した。次いで、この継手の溶接部の板厚1/4の位置から、ボンド部をVノッチ位置とするシャルピー衝撃試験片を各3本ずつ採取し、−60℃でシャルピー衝撃試験を行い、吸収エネルギー(vE−60)を測定し、それらの平均値を求めた。
<島状マルテンサイトの面積測定>
溶接熱影響部の断面を2段エッチングして島状マルテンサイトを現出させた後、2相域に加熱されるボンド部近傍を、走査型電子顕微鏡(SEM)を用いて倍率3000倍で10視野撮影し、画像解析して、島状マルテンサイトの平均面積を測定した。
No. shown in Table 1. After steel 1-29 was melted and made into a steel material (slab), it was heated under the conditions shown in Table 2 and subjected to hot rolling with a cumulative rolling reduction of 50-90%, resulting in a plate thickness of 50- The steel plate was made into a 150 mm steel plate, and the thick steel plate was quenched and quenched as it was, or allowed to cool and then reheated and then quenched and quenched, and then subjected to a tempering treatment. 1-33 product steel plates were produced. The steel plate thus obtained was subjected to the following test.
<Tensile test>
A JIS No. 4 tensile test piece with the rolling direction as the tensile direction was taken from the position of the thickness ¼ of each steel plate, and the tensile test was performed to measure the yield strength and the tensile strength (TS).
<Impact test>
Three V-notch Charpy impact test pieces with the rolling direction as the longitudinal direction were sampled from the position of the thickness ¼ of each steel plate, and Charpy impact test was performed on each test piece at −60 ° C., and the absorbed energy (vE -60) was measured and the average value thereof was determined.
<Impact test after welding>
Two welding test pieces collected from each steel plate were subjected to X groove processing (groove angle 45 °), and then subjected to submerged arc welding with a heat input of 50 kJ / cm to produce a multilayer welded joint. Next, three Charpy impact test pieces each having a bond portion as a V-notch position were sampled from the position of the plate thickness 1/4 of the welded portion of this joint, and subjected to a Charpy impact test at −60 ° C., and the absorbed energy ( vE-60) was measured and the average value thereof was determined.
<Area measurement of island martensite>
The cross section of the weld heat affected zone is etched in two steps to reveal island martensite, and then the vicinity of the bond portion heated in the two-phase region is 10 times with a magnification of 3000 times using a scanning electron microscope (SEM). A field of view was taken and image analysis was performed to measure the average area of the island martensite.
上記の試験結果を表3に示した。この結果から、鋼の成分組成が本発明に適合する発明例の鋼板(鋼板No.1〜25(ただし、No.1,2は参考例))は、いずれも母材の降伏強度が630MPa以上、引張強さが720MPa以上、母材のvE−60が120J以上であり、母材の強度・靭性に優れていることがわかる。さらに、溶接部の靭性vE−60は70J以上の靭性を有しており、溶接部の靭性にも優れていることがわかる。
これに対して、本発明の成分組成を外れる比較例の鋼板(鋼板No.21〜29)は、母材の降伏強度が630MPa未満、引張強さが720MPa未満、靭性vE−60が120J未満もしくは溶接部の靭性vE−60が70J未満のいずれか1以上であり、母材の強度・靭性および溶接部の靭性のいずれか1以上の特性が劣っている。また、個々の成分組成は適正範囲でも、(1)式を満たさない鋼板(No.28)は、島状マルテンサイトが大きくなっている。
また、表3の鋼板No.30〜33に示すように、鋼の成分組成が本発明に適合する鋼板でも、焼入条件や焼戻条件が本発明に適合していない場合には、母材の強度・靭性のいずれか1以上の特性が劣っていることが認められる。
The test results are shown in Table 3. From these results, the steel sheets of the invention examples (steel plates Nos . 1 to 25 (where Nos . 1 and 2 are reference examples) ) in which the component composition of the steel conforms to the present invention have a base material yield strength of 630 MPa or more. The tensile strength is 720 MPa or more, and the base material vE-60 is 120 J or more, indicating that the base material is excellent in strength and toughness. Furthermore, it can be seen that the toughness vE-60 of the welded portion has a toughness of 70 J or more and is excellent in the toughness of the welded portion.
On the other hand, the steel plates of comparative examples (steel plates Nos. 21 to 29) that deviate from the component composition of the present invention have a base material yield strength of less than 630 MPa, a tensile strength of less than 720 MPa, and a toughness vE-60 of less than 120 J or The toughness vE-60 of the welded portion is any one or more of less than 70 J, and any one or more characteristics of the strength / toughness of the base material and the toughness of the welded portion are inferior. Moreover, even if each component composition is an appropriate range, the steel plate (No. 28) which does not satisfy Formula (1) has large island martensite.
In addition, steel plate No. As shown in 30 to 33, even if the steel component composition conforms to the present invention, if the quenching condition or tempering condition does not conform to the present invention, one of the strength and toughness of the base material is selected. It is recognized that the above characteristics are inferior.
Claims (4)
記
Mn+Ni+Cr−12.5×C≧2.6(mass%) ・・・(1)
ここで、上記式中の各元素記号は、その元素の含有量(mass%)を示す。 C: 0.039 to 0.2 mass%, Si: 0.05 to 0.3 mass%, Mn: 0.5 to 5 mass%, P: 0.015 mass% or less, S: 0.005 mass% or less, Cr: 3 mass %: Ni: 5 mass% or less, Ti: 0.005 to 0.02 mass%, Al: 0.004 mass% or less, N: 0.007 mass% or less, B: 0.0003 to 0.003 mass%, In addition, when Mn, Ni, Cr and C satisfy the following formula (1), the remainder has a composition composed of Fe and inevitable impurities, and heat input is applied to multilayer welding with a heat input of 80 kJ / cm or less A high-tensile steel plate having an average area of island martensite of 3 μm 2 or less and a thickness of 30 mm or more .
Mn + Ni + Cr-12.5 × C ≧ 2.6 (mass%) (1)
Here, each element symbol in the above formula indicates the content (mass%) of the element.
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